STRUCTURAL COLORS I N INSECTS. I BY CLYDE
w.
MASOXI
Introduction On looking over the literature on insect colors, one finds a very striking lack of agreement on many points; the nature of various colors thought to be due to structural conditions has been the subject of a great many conflicting papers. The situation is hopeful, in that most people do not need to be convinced that structural colors are of widespread occurrence in the insect world, but in many instances there has been an unfortunate tendency to assume that all these colors are to be explained on the same basis. Actually, to determine the relationship of the structural features to the color phenomena observed in any given case requires more than a cursory inspection, particularly since pigmental and structural colors are combined in many instances and mutually influence the appearance observed. In a study2of structural colors in feathers the main criteria for the identification of different types of color were assembled, and their application to a number of typical cases was illustrated. The present paper is an attempt to carry out a similar study in the case of a sufficient variety of insect color types to show how such criteria may be applied to any given specimen. It is not the aim of the writer to prksent a mass of information regarding a large number of different insects, or to assume the position of feferee in all disputed cases. Such material, where it does appear, is incidental to the main purpose of this investigation, namely the development of reliable and simple methods for distinguishing between pigment colors and the different types of structural colors, when present in insects, either singly or in combination, It is hoped that such methods may be applied by entomologists to any cases which are of particular interest. The speciments used in the present work (furnished through the kindness of Dr. W. T. M. Forbes, of the Entomological Department at Cornell) were chosen irrespective of genus or species, the aim being to secure as great a variety of types of color as possible-in other words, to “sample” the collection on the basis of color phenomena rather than entomological relationships, though, naturally, representatives of different orders are present in the assortment which was studied. Such an effort is by no means original with the writer. Many other workers3 have made extensive studies with more or less the same purpose in mind, but some of their findings are none loo conclusive, and have placed The investigation upon which this article was based was supported by a grant to Messrs. Bancroft, Chamot and Merritt from the Heckscher Foundation for the $dvancement of Research, established by August Heckscher a t Cornell University. 2Mason: J. Phys. Chem., 27, 201, 401 (1923). a Hagen: Proc. Am. Acad., 17, (1881-82); Coste: Entomologist, 23, 24, (1890-91); Tower: Dec. Pub. Univ. Chicago, 10,33 (1903); Mallock: Proc. Roy. SOC.,85A,598 (1911); Biedermann: Handbuch vergl. Physiol., 3, I, 2, 1657 (1914); Onslow: Phil. Trans., 211B, I (1921); Suffert: Z. Morphol. Okol. Tiere, 111, 172 (1924).
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emphasis on the specimen rather than the method, so a somewhat different line of attack is perhaps not out of order. Much that appears in the following pages is not new, but inasmuch as it either confirms or contradicts earlier observations, its inclusion seems justified, particularly since it must be taken into consideration in any conclusions which may be drawn. Distinction between Structural and Pigment Colors To decide whether structure or pigment is the cause of a given color phenomenon, a number of tests are useful. Obviously, if some specialized structure functions in accordance with certain optical principles to produce color, the chemical nature of the tissue in which this structure is present will be of negligible importance, except in so far as it may render the physical structure susceptible to alteration by chemical reagents; but any treatment which alters the nature of the optical system will produce a modification of the color. On the other hand, if the color is due to selective absorption or reflection of certain of the constituents of white light by a pigment, it will be very little affected’by alterations in its physical condition, but will be susceptible to any chemical treatment which alters its inherent nature. In accordance with the above principles, a structural color should satisfy most of the following general criteria: Hue altered by pressure, distortion, swelling or shrinking. Color destroyed if system is made optically homogeneous by permeating with a liquid of the same index of refraction as that of the tissue, and color restored on removal of this liquid. Color not destroyed by bleaching, or affected by chemical reagents unless these swell, shrink or destroy the tissue. No pigment, except possibly black or brown, extractable by solvents or revealed by chemical tests. All constituents of the incident white light accounted for, either in the reflected, transmitted, or scattered light. Similar color phenomena duplicable with colorless substances in similar physical condition. Iridescence of some sort generally exhibited. The converse of the above will serve to characterize pigment colors, and usually the distinction can be made even more positive by further tests regarding the particular sort of pigment or structure concerned. Structural conditions may modify the apFearance of pigmented tissues, while the lustre of the surface, or the presence of over- or under-lying colored tissues may also have considerable influence; a given pigment may exhibit different tints or shades, or even apparently slightly different hues, depending on whether it is present in a fine or course state of subdivision, or in combination with white. In most cases, even where the type of color is established, study under a Greenough-type binocular microscope a t moderate magnifications will show the relative importance of these different factors in a most striking manner.
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For instance, there is probably about the same amount of pigmentation in all the black beetles, but those which have highly lustrous surfaces appear much blacker than those which have their integument dulled and roughened by age and exposure. Supplying the “glaze” by means of a film of oil or varnish gives a deep lustrous black in such cases. However, even such glossy black is not as dark as the black of certain butterflies. These latter do not even show “highlights”, yet their pigment is probably no more dense than that of the beetles, Their dark scales, naturally of little lustre and with numerous interstices between them, serve to “entrap” the light, and give a much darker shade for a given amount of pigment (just as velvet appears darker than satin dyed with the same dye). This appearance is particularly marked if one views the butterfly’s wing from the tip, that is, looking down into the shadows between the scales. It is highly probable that the browns and blacks are due to a melanin pigment, as they are in birds and mammals, the color present depending not only on the amount of pigment present but also upon its condition, black being found with granular pigmentation and brown with more or less perfectly diffuse pigmentation (pigment in solution rather than suspension in the tissue), By permeating the tissue with a colorless liquid of nearly the same index of refraction (say 1.55-1.6for chitinous material) the effect of any structures associated with the pigment to be studied is minimized, and the true absorption color may be observed by transmitted light, Besides the ordinary colors which depend upon absorption of light, certain others have been ascribed to pigments: white, which depends on the power of scattering light, and certain cases of iridescent colorings, which are said to be due to pigments showing selective reflection. The latter will be considered in connection with the discussion of other iridescent colors with which they are most likely to be confused.
White in Insects White depends for its perception on a rather high intensity of reflected or refracted colorless light reaching the eye of the observer without the perception of transparency or vitreous or metallic lustre. This will be the case whenever the surface by which such light is reflected or refracted to the eye are so small as not to be readily resolved as separate by the means of observation employed. Obviously, the finer the surfaces, the more perfect the “scattering” of light and the whiter the object. White objects, because of their power to scatter light in all directions, transmit light very imperfectly, and are more or less opaque, even though the actual material composing them is transparent (e.g. : snow and ice; foam or fog and water; sand and rock crystal; blanc fixe and barite; chalk and Iceland spar; “Rayonj’and “Cellophane”, and so on). The colorless metals appear white by reflection from irregular surfaces but in the case of all other colorless substances, refraction also plays a part in scattering the light to produce white, and the elimination of the reflection and refraction a t their numerous small bounding surfaces results in transparency, with no
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whiteness. Since this reflection and refraction by transparent materials depend upon the relative indices of refraction of the substance and the medium surrounding it, it is obvious that if the latter can be varied, (as it can in twophase systems when at least one phase is fluid) the white may be completely destroyed, by rendering the system optically homogeneous. So whenever we have practically colorless chitin in such form as to present many more or less unordered reflecting surfaces, we shall observe whiteness. Nearly all of the whites of insects are to be explained on this basis, as proved by the fact that when the air in contact with the chitin is replaced by a liquid of index of refraction near I. 5 5 they are rendered colorless and transparent, while if the liquid is either of greater or less index of refraction the destruction of the white is not so complete. On removal of the permeating liquid, the original whiteness is restored, proving that no white “pigment” has been dissolved out. The above procedure may readily be followed under the microscope, and the connection between air as a permeating medium, and the whiteness is very evident by reflected light or dark field illumination. The increase in transparency as the liquid penetrates the interfitices of the structure is very striking, and corresponds to the familiar process of “clearing” tissue for microscopic study. Ry means of the “Becke test” (given in any text-book of petrography) one can select from a series of liquids one which matches the index of refraction of the chitin almost perfectly, and thus render it almost completely invisible, even by dark field illumination, The scales of white butterflies and moths illustrate this very nicely. The question as to whether the whites of insects are due to pigment or to structure is merely one of definition, All white materials, whether commonly thought of as pigments or not, owe their whiteness to the reasons given above, and white paint is just as much a structural white as snow. We are ordinarily in the habit of thinking of pigments as something distinct from the substance which is pigmented by them, and from this point of view there are very few white pigments in nature. Generally the tissue in question is its own white pigment, and the white cannot be removed without destroying the structure on which it depends. Certain butterflies, notably the Pzeridae are said to owe their whiteness to a definite pigment (uric acid) which is present in their scales as an excretion from one stage of their deve1opment.l Although uric acid is white, and might well serve as a pigment when fincly divided, it plays a very minor part in producing the white of butterfly scales, for these may be extracted with solvents for uric acid (such as dilute alkalies) but on washing and drying their white appears undiminished. The scales become transparent on penetration with liquids of n = I. j j-1.6. Assuming that any uric acid is present in the free state, and not in solid solution in the chitin, it might be expected to appear white when finely divided, but the ribbed and reticulated structure of the scales accomplishes this so well that whether uric acid is present or not is of little importance, as far as their whiteness is concerned. Hopkins: Phil. Trans., 186, 661 (1896).
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The belief that the chalky whites of butterflies, particularly the Pieridae, are due to pigment is common; but the more lustrous whites, such as those of Ezrproctis are ordinarily admitted to be air-filled cavities in the scales. Actually both sorts have hollow scales, as do insects in general; both are rendered transparent by permeation with liquids of n = 1.55-1.6.~ and there is no question but that, whatever may be the chemical composition of the scale in both cases, the white is due to essentially the same causes, namely, the scattering of light by reflection and refraction by the minute structures of the scales.
Lustrous Whites The question of lustre is closely related to that of color, and may best be discussed in connection with white, for we have all types illustrated by colorless insects. White butterfly wings exhibit lustres ranging from chalky to micaceous, silvery, or pearly, and these are of necessity to be explained on the basis of the structures present, since the constituents of the wings are essentially the same in all these cases, Chalky or matte lustre is characterized by the absence of high lights, and by practically uniform scattering of light in all directions. This requires a random arrangement of the surfaces which reflect and refract the light. Although the scales of butterflies are arranged systematically on the wing membrane like shingles on a roof, this is not of importance in the case of the chalky whites, for the individual scales serve to scatter light in all directions, whatever their arrangement may be; a single scale appears just as white as do a group of scales. The overlapping of the scales on the wing contributes little to their whiteness, because of the high degree of opacity of the white. Such scales, under the microscope, show more or less regular longitudinal striations, boldly defined, and probably due to deep corrugations in the surface of the scale, or to longitudinal partitions in it, or to both, Besides these, fine transverse markings are present, which, although unordered i n their pattern, add materially to the opacity and whiteness. On permeating with a liquid of n = 1.55-1.6 the markings and outline of the scale become almost invisible. The above multiplicity of fine structural details furnishes reflecbing and refracting surfaces to scatter the maximum amount of light in all directions, and to give the minimum of sheen. Intermediate between the matte whites and the pearly or silvery whites is that shown by Euproctis. Here a distinct sheen is observable, when the line of vision is that direction along which light would be reflected by the surfaces of the scales. In other positions the wing appears matte white, tho not so bright or opaque as in the case of the typical chalky whites; single scales appear very much more transparent and scatter less light than when grouped on the wing, where under the Greenough they are seen to overlap considerably; this overlapping adds to their reflecting power, for if a blackened scalpel is inserted under the free edge of the scales they permit it to be seen thru them very clearly. With high powers, the individual scales are seen to be striated longitudinally, but these markings are very much less distinct than in the scales of the Pieridae, and constitute the only surfaces for scattering light, the
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finer reticulations being absent. The whole appearance is that of considerable transparency, even without a permeating liquid; when the scales are filled with liquid of n = I . j5-1.6 they are barely visible under the microscope, and the entire wing is rendered so transparent that one may easily read thru it, even when the liquid chosen does not match the index of refraction of the scales very perfectly. In the above case, the white is due to a certain amount of scattering by the finer structures of the scales but we have here also the necessary structure for the production of pearly lustre, namely, the superposed transparent reflecting surfaces presented by the scales as they lie on the wing. This is the situation which is present in all cases of pearly lustre, such as mother-of-pearl, ground mica, crystallized CdI2, or a pile of glass plates1, and it is of considerable importance also in the production of metallic lustre2. The essential difference between matte whites and pearly whites depends on the amount of random scattering which the presence of minute structures in the scales may cause. When the lamellae which produce pearly lustre are almost perfectly plane and transparent, and there is a minimum of diffuse scattering of the light, we have very perfect and typical pearly lustre, which may under favorable circumstances verge into true metallic lustre. This is exhibited by Helicopus cupido and by the common “Silverspot” (Argynnis). The name of the latter is very appropriate, for the spots, though not as metallic as burnished silver, strikingly resemble matte silver. They do not scatter light well, and only along the line of direct reflection do they appear bright. Even in this position they do not send as much light to the eye as do the chalky white scales. Microscopic study of the scales shows longitudinal striations and a distinctly transparent appearance. There is some scattering of the light which falls across the striations, but on the whole a rather glassy appearance is noted by reflected light. The pearly areas of Helicopis cupido are similar in appearance to those of the “Silverspot”, but are even more highly lustrous and show distinct though pale iridescent colorings, which strongly resemble those of mother-of-pearl. The overlapping, transparency, and lack of light-scattering power or whiteness are even more pronounced; microscopically the scales show only very faint striations and appear t o be little more than a transparent envelope with thin walls nearly in contact, all of which constitutes a system for the production of pearly or metallic lustre by superposed laminae, more perfect than that of Euproctis because of the almost complete transparency and absence of diffusion of the light. However, even here the whiteness and reflecting power a t the most favorable angle are rather less than in the matte whites. Leydig3 has ascribed the pearly lustre of Argynnis to the fine structure of the scales, without specifying how this is related to the general phenomenon Bancroft and Allen: J. Phys. Chem., 28, 546 (1924); 29, 564 (1925). ‘‘Silver” patterns on the wall paper are made with ground mica. a roll of waxed paper is pearly, while a roll of colorless, transparent “cellophane” (sheet $iscose) is hardly to be distinguished from a polished steel rod. 8Arch. Mikr. Anat., 12, 536 (1876).
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of pearliness, and Biedermann' while disagreeing with him on details, apparently also failed to recognize that superposed, more or less transparent lamellae were in themselves sufficient to account for all the observed lustre. Siiffert2 also places undue emphasis on the minute structural details of the scales, and considers their high reflecting power to be due to the longitudinal ridges of the upper lamella, which serve by two refractions and one total reflection to send the light back along the original direction, in somewhat the same manner as does ribbed glass. He admits that such a structure must be highly perfect if it is to function as indicated, but apparently does not appreciate the possibility of the high lustre being accounted for by any less complex system. Microscopic study of scales of Argynnis, with transmitted and with vertical illumination reveals a structure that is closely similar to that described by Suffert: a ribbed upper lamella, a fairly smooth lower lamella, an interior space with fairly plane surfaces and a relatively small amount of spongy or columnar material in this cavity. Slow permeation by viscous liquids aids in the interpretation of these appearances, and helps to isolate their several effects; for instance, the iridescent colors seen by axial reflected light are observed only when the interior of the scale is empty, irrespective of whether the lamellae are in contact with liquid outside, The dry scale is apparently the same viewed either side up, If the scales are covered by a cover glass, and a viscous liquid is allowed to surround them, one notices that the reflecting power of unwetted scales is apparently no greater than that of the air spaces between the slide and the cover glass. Also, if one compares the reflecting power of the scales on the wing with that of a single thin film of glass, the latter is much more brilliant, yet when we consider that the scales consist of two lamellae, separated by an air space, and that they overlap so that they are two deep on the wing, it is evident that we might expect an even higher degree of reflection from the four films of chitin thus functioning as reflectors. Actually it appears that the high lustre of the pearly scales is in spite of rather than because of their fine structure. The latter plays its part in diffusing the light, so that the metallic appearance is visible over a wider angle, and in producing certain color phenomena, but acts to cut down the efficiency of the multiple-film system for producing metallic or pearly lustre; this loss in specularity of reflection is accompanied by an increased appearance of whiteness, and greater opacity of the scales. The delicate iridescent colorings, the cause of which will be discussed later, enhance the resemblance to pearl, and possibly give the illusion of even greater lustre. Non-Porous Whites In addition to the ordinary sort of structural whites in which the minute surfaces are in contact with air, another type may occur in which the structure is non-porous, and possesses no air-filled spaces. The reflection and refraction Handbuch vergl. Physiol., 3, I, 2, 1932 (1914). 210c. cit. p. 287. Z. Morphol. Okol. Tiere, 111, 172 (1924).
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which are the cause of the scattering of the light take place at surfaces bounding two tissue materials of different refractive indices, and their perfection will be governed by the amount of this difference. Such a system is optically the same as a porous white rendered partially transparent by a liquid of index of refraction fairly near that of the tissue but not matching it; examples are common : marble, vitrified porcelain, ivory, pearl, paraffin wax, etc. Obviously, any attempt to destroy the white by rendering it transparent by means of a permeating liquid cannot be successful, for there are no interstices to be filled, The only possibility would be in some way to render the indices of refraction of the various structures identical, which would involve chemical alteration, hydration or some similar treatment. The non-porous whites are of very little importance in the insect world, and are mentioned merely for the sake of completeness; the “air whites” may definitely be said to be the only important cause of whiteness in insects. In most cases whites of this sort are more or less transparent, and of little scattering power; in insects this is particularly true, because the various kinds of material which make up the tissues have almost the same index of refraction; in no case is the difference in the indices nearly as great as the difference between one of them and that of air. Wing membranes, when bare of scales, and occasionally veins may give a weak white, but these play a very small part in the whiteness of the insect, as a whole. Influence of Structure on Coloration Where a color pigment is combined with white it is usually dissolved or dispersed in the tissue itself, SO that we have to deal with a more or less colored and transparent substance, which is in a state of subdivision corresponding to that existing in the whites. Unless the tissue is very heavily pigmented and rendered almost opaque, such fine structure will reflect a large part of the incident light; what light does actually enter the colored tissue will be refracted and reflected out again, without traversing more than a very short distance in the tissue. Thus there will be little absorption, although the scattering of the light takes place just as in the unpigmented tissues and as a result we have the hue of the pigment with a large admixture of white, giving a tint much lighter than the tissues would exhibit if the fine structure were absent. Many common materials illustrate this great difference in tint dependent upon their fineness; beer and its foam, vulcanized rubber and its dust, the “streak” of colored minerals, taffy before and after pulling, butter and cream, coarse and fine copper sulphate, and SO on. It follows that, since structural conditions cause a very much lighter tint than shown by the colored substance in mass, the best way to observe the true hue and tint is by eliminating reflection and refraction as completely as possible. This is accomplished by rendering the system optically homogeneous by bringing a liquid of the proper index of refraction in contact with all the minute surfaces, just as is done when whites are rendered transparent. The above treatment is particularly useful in studying the hue, concentration, uniformity, and state of aggregation of pigments in the scales of butter-
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flies and moths, and one may rest assured that any color observed by transmitted light iinder these circumstances is due t o pigmentation and not to any structural effect. To the naked eye, permeation with liquids darkens the tint considerably and increases the transparency of the scales and of the wing as a whole. This behavior has many familiar analogues; wet and dry watercolors, grease or water spots on wood, colored fabrics, and papers, etc., mud and dust; copper sulphate may be powdered so fine as to be almost white, but its deep blue is restored by wetting with a liquid of the proper index of refraction. M'here pigmentation is combined with some specialized structure other than that capable of producing ordinary white, a number of effects are possible. Hairs and scales play a considerable part jn modifying the tint and lustre of the pigment. The deep blacks and browns of some of the bees are closely analogous t o the rich shades of velvet or plush; the hairs standing on end have their surfaces in position to reflect a minimum of light, and if they are even moderately pigmented appear in a very much darker shade than if they are lying more or less flat, (as in crushed velvet). If the scales are all practically in the same plane, they present an excellent reflecting surface to light which falls along their length; this corresponds t o the structure of satin, and, like it, may exhibit a very high lustre. Ornithoptera poseidon exhibits two types of black which illustrate the influence of structure on pigment coloration. The deep velvety black which occupies most of the non-iridescent portion of the wing is due to the heavily pigmented, overlapping, convex scales, of low reflecting power, and with only a small portion of their curved surfaces in position to reflect light at any given angle. The brownish black areas just in front of the rear iridescent border of the upper sides of the forewings are markedly different in appearance; the scales are very much narrower and more numerous, stand erect, are sharply curved at the upper end and less heavily pigmented. They overlap very little, and permit the light colored wing membrane to be seen between them; under the microscope the apGearance resembles that of open-weave Rrussels carpet. Individually they show more reflecting power than do the scales from the deep black portions, and on the wing this is more pronounced because of the greater number of minute surfaces presented by the finer scales, and because of the partial visibility of the wing membrane between them. It has been pointed out that relatively transparent, smooth, overlapping scales may show a pearly to metallic sheen; if these are colored this effect is greatly enhanced, and may be of typical metallic lustre1. The wings of Chrysophanus virgaureae and their members of this family have the appearance of tarnished brass, but their scales are as described above, with the addition of a yellow orange pigment. Dyeing the silvery scales of Helicopus cupido or il pile of three to five sheets of yellow gelatine has a metallic lustre equal to that of polished brass, and corresponding appearances may be obtained with other colors. Crystallized lead oxide looks golden; anhydrous chromium sesquichloride is a metallic purple; ground phlogopite resembles bronze; the goldfish is golden rather than yellow because of his scales.
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A r g y n n i s or the pearly ones of Euproctis with dilute Bismarck brown gives them a golden appearance. This metallic appearance of relatively smooth, transparent scales is observed irrespective of whether their color is due to pigment or to structure, and Urania, Morpho, Apatura, in fact most of the brilliantly lustrous and iridescent butterflies may be cited as examples, without discussing the nature of their coloring at present1. Biedermann2 has discussed in considerable detail the relationship of the form and surface of insect scales to the color phenomena which they exhibit collectively on the wing of the insect. Convex, concave, warped, or stepped surfaces, or unusual inclinations of the scales may prevent their color (pigment or structural) from being seen except in certain directions. The blue-green bars on the upper surface of the wings of A p a t u r a (Chlorippe) seraphina are composed of scales which are strongly convex and form minute cylindrical surfaces, the axes of which are cross-wise of the scales. Some part of such surfaces is in position specularly to reflect light to the eye of the observer almost irrespective of the position of the wing as a whole, and the color bands may be seen from any position. The remainder of the upper surface of the wing appears dull brown, except in certain positions, where it is purple to blue-green, so that as the wing is inclined this brilliant color flashes out, only to disappear as suddenly with further inclination of the wing. Study with the Greenough-type binocular microscope permits this behavior to be observed at moderate magnifications with highly realistic stereoscopic effect, SO that interpretation of appearances is greatly simplified. It is found that the scales in these brown to blue areas are not particularly convex but are markedly inclined to the surface of the wing, so that the tip is considerably higher than the base, and the plane of the scale makes an angle of about 30’ with the plane of the wing. The reflection from these scales is specular,and as a consequence they appear bright only when the angle of incidence of the illuminating beam and the angle of reflection along the line of vision are equal3. These two factors combine to render the reflection color visible when one looks “along the grain” of the wing, while if the line of vision falls “against the grain” there is no reflection, and the effect is that of the brown pigmentation, the shade of which is darker because the scales are seen endwise and the interstices beneath them serve to “entrap” the light. All this may be checked varying the plane of the scales (“ruffling them up” or flattening them down) with respect to the plane of the wing, and study of single scales is also of value. Essentially the same situation exists in A p a t u r a ilia, and it is of interest to note that even the scales in the white bands possess the same blue reflections, which may be enhanced by dyeing them with a brown or black dye so that the 1 In discussing the influence of the form of the scales on the color exhibited by them, Onslow mentions the importance of flat, smooth surfaces in the production of metallic appearances. Phil. Trans., 211B,5 (1921). Handbuch vergl. Physiol., 3, I , 2, 1657 (1914). It should be remembered that if the plane of the scale makes an angle of 30’ with that of the wing, a vertical illuminating beam will be reflected a t an angle of 2 X 30’ from the normal to the surface of the wing. (A change of n degrees in the plane of the reflector causes a change of zn degrees in the direction of the reflected ray.)
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white is eliminated. Ancyluris meliboeus, Acraea sp., Callicore eluina, Zephyrus quercus, and numerous other butterflies show a bluish over-sheen or “bloom”, in addition to their ordinary pigment coloring (commonly melanin brown). Tyndall Blues in Insects The ability of a finely divided transparent substance to scatter light increases with increasing fineness until the order of magnitude of the particles is near the wave-length of light; for finer material the action is selective, and the particles scatter the short wave-lengths of white light, while permitting the longer wave-lengths to pass through unaffected. Such a system appears turbid blue to white from positions out of the line of the directly transmitted light and turbid yellow to red in this latter direction. Tyndalll first explained this phenomenon, and it has been recognized in a great number of common objects; the sky, blue eyes, smoke, “opal” glass, skimmed milk, blue feathers, very fine precipitates, collodion jelly, sublimates on charcoal in blowpipe analysis, etc. The properties of Tyndall blues have been discussed at some length in a previous paper2, and only the main criteria for their recognition need be summarized here: I-Minute structure, of dimensions less than o. jp, of different index of refraction from that of the surrounding medium. 2-Blue to bluish white scattered; brown to red, orange, 01 yellow by transmitted light. 3-Scattered light more or less polarized; vibrations in a plane perpendicular to the direction of the incident beam. To observe the scattered light, it is desirable that the illuminating beam should fall more or less tangentially on the tissue under examination, and if any surface irregularities are present, the scattering of light by these should be eliminated as completely as possible, preferably by immersion in a liquid of the same index of refraction as that of the tissue. Dark field illumination is very satisfactory for the examination of Tyndall blues, but the polarization is not readily observed unless the light is unidirectional. A cap nicol prism above the eyepiece, with or without a “Selenite plate” serves to determine the polarization of the scattered light. By transmitted light with critical illumination and an objective of at least N.A. = 0 . 8 j a thin layer of the material should show fine stippling, the individual particles being about 0 . 3 t~o 0.5p between centers. These may also be observed by reflected light under favorable circumstances. The color of the tissue is, of course, yellow to orange or brown, by transmitted light, unless dark pigment underlies the blue structure, and this may readily be noted. A dark background for the blue cuts off any stray light from below, so that only the scattered light is observed. Biedermann3 mentions Schatz as having ascribed the metallic blue of the Morphos to a turbid layer over a dark background and points out that the Phil. Mag., (4), 37, 385 (1869). J. Phys. Chem., 27, 215 (1923). Handbuch vergl. Physiol., 3, I, 2, 1970 (1914).
* Mason:
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lustre and brilliancy of the color are inconsistent with the thickness of the structure where they are assumed to originate, while the change of hue with changing angle of incidence is certainly different from the behavior of Tyndall blues. Needless to say, no evidence structural or optical, has been found to support Schatz’s supposition with respect to the metallic blues. In Enallagmae the blue of the abdomen lies over a dark layer, to which it is closely adherent, but it is possible t o dissect am-ay the underlying pigmented tissue, and to study the blue layer by itself. The criteria given above are all satisfied, although the polarization is not very distinct. In all probability this case of Tyndall blue is due to fine particles enclosed in a chitinous material, for the color is not destroyed by prolonged soaking in permeating liquids, and pressure has little effect on the blue, tho if pores were present they should be partially closed by this treatment.
Mesothemis simplicicollis is typical of dragon flies which develop a “bloom” with maturity; the color of the body of this insect is really only a very grayish blue, but it constitutes an example of Tyndall blue whcre air is in contact with the surfaces which scatter the light, This “bloom” or effiorescence may be removed by scraping, leaving exposed the black or brown integument which serves as a dark background for the blue. The scrapings are in such a fine state of subdivision that they appear almost opaque by transmitted light and scatter light very effectively. HOWever, their microscopic structure is indefinite and too coarse to give better than a grayish blue. When the “bloom” is wetted by a liquid of index of refraction near 1.5 it becomes completely transparent and invisible and the dark pigmented tissue beneath it is perfectly visible. On removal of the liquid the original appearance is restored.
A somewhat similar “bloom” is found on the wings of Libellula pulchella (“Ten Spot”), in the form of white cloudy spots, which shade to whitish blue at their edges. This is a Tyndall blue, the white portion of the spot being due to a coarsening of the structure. Microscopic examination shows the spots to consist of a very fine granular coating on the wing membrane; this may be scraped off readily, and is rendered completely invisible by wetting with liquids of n = I. j. Removal of the liquid restores the original appearance. It is noteworthy that while all non-metallic blues in feathers are Tyndall blues, in insects examples are few, and most blues are due to other causes. Of the possible specimens suggested by Dr. Forbes, only one was found to possess a blue pigment: the hairs of Xylocopa Caerulea are a clear blue by transmitted light, and show no evidence, structural or optical, of any but pigment coloration. The color change brought about by treatment with acids is undoubtedly of the nature of an indicator effect, more or less perfectly reversible by alkalies.
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The conclusions of this paper are as follows: I.
All whites of insects are structural.
2. Pearly and metallic lustre are due to superposed transparent parallel laminae.
3.
Tyndall blue occurs in a few insects.
4.
Blue pigments do not appear to be present in insects.
5 . Structural conditions may modify the appearance of pigment colors very markedly. Laboratoru of Chemical Microscopy Cornell University.
